1. Field of the Invention
This invention relates to FLADE aircraft gas turbine engines and, more particularly, to exhaust nozzles of such engines.
2. Description of Related Art
One particular type of variable cycle engine called a FLADE engine (FLADE being an acronym for fan on blade) is characterized by an outer fan connected to and thus driven by a radially inner fan and discharging its flade air into an outer fan duct which is generally co-annular with and circumscribes an inner fan duct circumscribing the inner fan. One such engine disclosed in U.S. Pat. No. 4,043,121, entitled “Two Spool Variable Cycle Engine”, by Thomas et al., provides a flade fan and outer fan duct within which variable guide vanes control the cycle variability by controlling the amount of air passing through the flade outer fan duct.
Other high performance aircraft variable cycle gas turbine FLADE engines capable of maintaining an essentially constant inlet airflow over a relatively wide range of thrust at a given set of subsonic flight ambient conditions such as altitude and flight Mach No. in order to avoid spillage drag and to do so over a range of flight conditions have been studied. This capability is particularly needed for subsonic part power engine operating conditions. Examples of these are disclosed in U.S. Pat. No. 5,404,713, entitled “Spillage Drag and Infrared Reducing Flade Engine”, U.S. Pat. No. 5,402,963, entitled “Acoustically Shielded Exhaust System for High Thrust Jet Engines”, U.S. Pat. No. 5,261,227, entitled “Variable Specific Thrust Turbofan Engine”, and European Patent No. EP0567277, entitled “Bypass Injector Valve For Variable Cycle Aircraft Engines”. U.S. Pat. No. 7,770,381, entitled “Duct burning mixed flow turbofan and method of operation”, discloses the use of airflow from a FLADE fan stage may be discharged into an interior space of an exhaust nozzle to cool the nozzle surfaces, and then ejected through a slot or in the exhaust nozzle to provide some supplemental thrust.
It is highly desirable to provide an engine with apparatus for introducing FLADE air into the exhaust nozzle airflow that maximizes thrust for a wide range of aircraft operation. Variation in the nozzle exit area is important for attaining high performance across the flight envelope, however, this is in conflict with airframe designers objectives to maintain a fixed structure for low observable and mechanical reasons. Thus, it is also desirable to provide a FLADE engine with fixed exit area and fixed outer structure and engine envelope.